This is a single centre, randomised, double blind, two-way crossover study, to investigate the safety, tolerability, pharmacokinetics and pharmacodynamics of inhaled dry powder GW685698X (800?g) containing magnesium stearate. Magnesium stearate is an excipient added to improve the physical stability of inhaled dry powder formulations. Male and female patients with mild/moderate asthma will be randomised to determine which order they receive a single inhaled administration of GW685698X (800?g) with and without magnesium stearate. There will be a wash-out period of at least 5 days between doses.